The present invention relates to wireless communications systems and methods, and more particularly, to systems and methods for providing services in wireless communication systems.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have been long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95,-lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a conventional terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. In a typical FDMA system, each of these discrete frequency bands serves as a channel over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell.
The limitations on the available frequency spectrum present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system may require more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of channels available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system having only several hundred allocated frequency bands.
Another technique which can further increase system capacity and spectral efficiency is the use of time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots. Communications over a frequency band typically occur on a repetitive TDMA frame structure that includes a plurality of time slots. Examples of systems employing TDMA are those conforming to the IS-136 standard, in which each of a plurality of frequency bands are subdivided into 3 time slots, and systems conforming to the GSM standard, which divides each of a plurality of frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during assigned time slots.
A channel in a TDMA system typically includes at least one time slot on at least one frequency band. Typically included among the channels in a TDMA system are dedicated control channels, including forward (downlink) control channels for conveying information from a base station to subscriber terminals, and reverse control channels for conveying information from subscriber terminals to a base station. The information broadcast on a forward control channel may include such things as a cell""s identification, associated network identification, system timing information and other information needed to access the wireless system from a subscriber unit and to manage radio resources in the system. Reverse control channels are typically used for transmitting access requests from subscriber terminals. A channel used for this purpose may be referred to as random access channel (RACH).
An exemplary slot allocation, in particular, one utilized by wireless systems complying with the IS-136 standard, is illustrated in FIG. 3. For groups of three repeating slots on the uplink and downlink carrier frequency bands used by a base station, a xe2x80x9cslot pairxe2x80x9d on one pair of uplink and downlink carrier frequency bands is reserved for the provision of a forward Digital Control Channel (FDCCH), and a reverse DCCH (RDCCH), with other slots being assigned to Digital Traffic Channels (DTCs).
As illustrated in 4, the FDCCH has a plurality of xe2x80x9clogical channelsxe2x80x9d mapped thereon, including a multiplexed Broadcast Channel (BCCH) designed to convey information about system configuration and system access rules, and a multiplexed point-to-point short message service (SMS), paging and access response channel (SPACH). The BCCH is further divided into a Fast Broadcast Channel (F-BCCH) for conveying time-critical information such as system identification (ID) and registration information, an Extended Broadcast Channel (E-BCCH) for conveying less time critical information such as neighboring cell lists, and an SMS Broadcast Channel (S-BCCH). The SPACH comprises a short message service channel (SMSCH) for carrying messages, a paging channel (PCH) for conveying system pages, and an access response channel (ARCH) for providing system response to queries from subscriber units and other administration information. The RDCCH is used to provide a Random Access Channel (RACH), which is used by terminals to transmit requests to access the wireless system.
Wireless systems typically provide control information over FDCCHs and RDCCHs. As illustrated in FIG. 5, a Layer 1 (Physical Layer) message transmitted over the FDCCH typically is constructed from a Layer 3 message that is broken down into Layer 2 frames, a respective one of which is transmitted during a respective slot after convolutional coding and interleaving. Each Layer 1 FDCCH message includes coded Layer 3 data, along with a synchronization information (SYNC) field and a Coded Superframe Phase (CSFP) field that indicates the position of the FDCCH slot in a Superframe.
The FDCCH message also includes a Shared Channel Feedback (SCF) field that contains information about the reservation status of an associated RDCCH RACH. The reservation status information in SCF field includes a Busy/Reserved/Idle (BRI) field that indicates whether the corresponding RDCCH RACH is busy, reserved or idle. A Received/Not Received (R/N) field indicates whether a RACH burst was received on the corresponding RACH. A Coded Partial Echo (CPE) field may be used to identify a terminal for which a RACH burst has been successfully received.
Wireless communications systems are often subject to environmental effects that can render system access difficult. A wireless call which could be placed under system operating parameters that are designed to produce an acceptable level of communications quality under a set of nominal environmental conditions, may not be possible under xe2x80x9csub-nominalxe2x80x9d conditions of fading, shadowing by intervening objects such as hills, and attenuation by distance and by structures such as buildings.
High-penetration messaging and paging solutions have been proposed that allow a base station to transmit a short alphanumeric message to a terminal in a disadvantaged location, such as in a xe2x80x9cholexe2x80x9d between coverage areas or within a building or tunnel, using a high-penetration control channel. In response to the receipt of such a high-penetration short message, the mobile terminal can transmit a similar high-penetration acknowledgment, and later move to a less disadvantaged location and call back the calling party identified in the short message. Examples of high-penetration messaging services are described in U.S. patent application Ser. No. 09/193,261(Rydbeck et al., filed Nov. 18, 1998), U.S. patent application Ser. No. 09/195,790(Rydbeck et al., filed Nov. 18, 1998), and U.S. patent application Ser. No. 09/195,315(Khayrallah et al., filed Nov. 18, 1998), each of which are assigned to the assignee of the present invention. However, because such high-penetration services may utilize spectral resources that might otherwise be used for voice traffic, there is a need for improved techniques for efficiently providing such services.
In light of the foregoing, it is an object of the present invention to provide apparatus and methods for efficiently providing regular and high-penetration services in wireless communications systems.
This and other objects, features and advantages are provided, according to the present invention, by apparatus (systems) and methods in which a plurality of base stations (e.g., omnidirectional base stations, base stations serving multiple sector cells, or similar transmitter/receiver units) of a wireless communications system includes a subset of base stations that communicate with terminals over both regular control channels, e.g. Digital Control Channels (DCCHs), at a first redundancy level and over high-penetration control channels, e.g., high-penetration Digital Control Channels (HP-DCCHs) at a second redundancy level greater than the first redundancy level. For example, cells (sectors) served by the base stations of the system may be organized into groups, such as frequency or code reuse groups, such that a respective group of base stations serves a respective group of cells. The subset of base stations providing high-penetration control channels may include at least one base station from each group, such that high-penetration services may be effectively provided without using an inordinate amount of channel capacity for high-penetration control channels. Base stations providing high-penetration control channels may also be distributed more densely than the groups of cells in order to provide enhanced coverage in obstructed locations such as interior spaces in buildings. According to another aspect of the present invention, higher capacity high-penetration messaging may also be provided over a traffic channel for a terminal accessing the wireless system via a high-penetration control channel. According to yet another aspect of the present invention, candidate regular and high-penetration control channels may be identified in Neighbor Cell Messages transmitted over regular and high-penetration control channels to guide terminal scanning and control channel camping.
In particular, according to the present invention, a wireless communications system includes a plurality of base stations that communicate with terminals over physical channels. The plurality of base stations communicates control information with terminals at a first redundancy level over regular control channels mapped onto the physical channels. The plurality of base station includes a subset of base stations that also communicate control information with terminals at a second redundancy level greater than the first redundancy level over high-penetration control channels mapped onto the physical channels.
According to an aspect of the present invention, the subset of base stations providing both regular and high penetration control channels is distributed to efficiently provide service without using inordinate channel capacity. A base station of the subset of the plurality of base stations communicates over a regular control channel with terminals located in a first coverage area and communicates over a high-penetration control channel with terminals located in a second coverage area larger than the first coverage area. In one embodiment, cells served by the plurality of base stations may be grouped into a plurality of groups, such as frequency reuse or code reuse groups, such that a respective group of base stations serves a respective group of cells. The subset of the plurality of base stations may include at least one base station from each group. The base stations of the subset of the plurality of base stations may also be geographically distributed with a greater density than the groups to thereby provide improved service to obstructed locations.
According to another aspect of the present invention, a base station of the plurality of base stations provides a voice service to a terminal accessing the system via a regular control channel, and wherein a base station of the subset of the plurality of base stations provides a message service to a terminal accessing the system via a high-penetration control channel. The message service may be provided over a high-penetration forward control channel, or may be provided over a traffic channel.
According to another aspect of the present invention, a terminal for communicating with a wireless communications system over physical channels includes a receiver operative to receive control information transmitted over a plurality of regular control channels and high-penetration control channels, the receiver operative to receive control information at a first redundancy level over the regular control channels and to receive control information at a second redundancy level greater than the first redundancy level over the high-penetration control channels. A controller is operatively associated with the receiver and operative to scan the receiver over the plurality of regular and high-penetration control channels and to camp the receiver on a selected regular or high-penetration forward control channel based on a measure of communications quality.
In embodiments according to this aspect, the receiver is operative to receive information over a regular or a high-penetration control channel that identifies a plurality of candidate regular and high-penetration control channels. The controller is operative to scan the receiver over the plurality of candidate regular and high-penetration control channels. The controller may be operative to transition the receiver to camping on one of the candidate regular control channels when communications quality for the one candidate regular control channel meets a predetermined criterion. The controller may also be operative to transition the receiver from camping on a first regular control channel to camping on a second regular control channel selected from the plurality of candidate regular and high-penetration control channels when communications quality for the second regular control channel exceeds communications quality for the first regular control channel. The controller may be further operative to transition the receiver from camping on a regular control channel to camping on a high-penetration control channel selected from the plurality of candidate regular and high-penetration control channels when communications quality for the regular control channel and for the candidate regular control channels of the plurality of regular and high-penetration control channels is unacceptable. The controller may also be operative to transition the receiver from camping on a high-penetration control channel to camping on one of the candidate regular control channels when communications quality on the candidate regular channel meets a predetermined criteria.
Operating methods for wireless communications systems and terminals are also described. Improved wireless communications services may thereby be provided.